This invention relates to daylight fluorescent lamps and the two phosphor components incorporated therein. More particularly, it relates to lamps containing two phosphor components that yield higher lumens per watt with a suitable color rendition than prior standard daylight lamp.
The color characteristics of light emitted from a fluorescent lamp depends on the choice of phosphors used to coat the internal walls of lamp envelope. Emission spectra of luminescence centers in most phosphors consist of a single band peak at one particular wavelength. Therefore, in order to have white light it is necessary to either apply a mixture of phosphors or use a single phosphor containing more than one kind of luminescent center (such as the alkaline earth halophosphates). It is not enough to obtain the desired chromaticity coordinates and there are an infinite number of possible combinations of bands that would result in the same set of coordinates. It is also necessary that the lamp produce an acceptable luminous flux (brightness) and satisfactory optimum color rendition for all regions of the visible spectrum.
There are four standard lamps used today, daylight, cool white, white, and warm white and the desired chromaticity coordinates for these lamps are given hereinafter.
While it is possible to determine by theoretical computations the spectral energy distribution for a theoretical blue component and a theoretical yellow component that upon being blended together will yield a lamp having either brightness or color rendition maximized, such theory has to be tailored to the restraints as they exist in nature. In theory, a combination of a line emitting blue component and a line emitting yellow component would yield a lamp having the maximum brightness. Such a lamp however, cannot be produced for a number of reasons. First, phosphors having a line emission do not exist. Secondly, even if they existed the color rendition would be extremely poor because only two colors would be emitted and would result in color distortion in the area lighted by the lamp. Until recently the primary emphasis was placed upon color rendition with a suitable brightness. The single component halophosphates having two luminescent centers have been used to produce the aforementioned four white colors. The energy shortage, however, has shifted the emphasis to maximize lumens per watt of energy with an acceptable color rendition thus enabling a lower energy input to achieve the same level of brightness. While in theory, a two-component blend can produce warm white, there is no known binary combination of lumiphors that yield that color, however, an advantageous combination to produce a lamp having a chromaticity equivalent to daylight has been discovered.
It is believed, therefore, that a two-component phosphor system that takes into account the variables of brightness, color rendition and the lumiphors that exist in nature which when blended together will result in emitting light that corresponds to the standard daylight color and which maximizes the lumens per watt of energy input is an advancement in the art.